This invention relates to Zero Index Materials (ZIMs) (as defined herein) and significant improvements to omnidirectional reflector devices and hollow waveguides.
There have existed previously materials known as NIMs (Negative Index Materials). As their name connotes, NIMs are materials having a negative optical index of refraction. Such materials are not known to occur naturally but their theoretical possibility was shown by Veselago in 1968 [V. G. Veselago, Sov. Phys. Usp., vol. 10, p. 509 (1968)]. Realization of an actual NIM waited until 2000 when D. R. Smith and co-workers [D. R. Smith, et al., Phys. Rev. Lett., vol. 4, p. 4184 (2000)] experimentally tested an artificially constructed NIM, a so-called metamaterial, consisting of an array of split ring resonators proposed by Pendry and co-workers in 1999 [J. B. Pendry, et al., IEEE Trans. Microwave Theory and Tech., vol. 47, p. 2075 (1999)]. Subsequently, the development of NIMs has occurred at an astounding pace due to the potential applications (see, for example, [D. R. Smith et al., Science, vol. 305, p. 788 (2004)]). NIMs have the property of refracting light in the opposite way with respect to which an ordinary material does [V. G. Veselago, Sov. Phys. Usp., vol. 10, p. 509 (1968)] and it was predicted that they could be used to construct a “perfect” lens, i.e., a lens that can focus all Fourier components of a two dimensional (2D) image, even those that do not propagate in a radiative manner [J. B. Pendry, Phys. Rev. Lett. vol. 85, p. 3966 (2000)].
Until now omnidirectional reflector devices have been mainly designed by using the electromagnetic gap that is formed in periodic or quasi-periodic photonic band gap (PBG) structures made of positive index materials. (See, for example, [E. Yablonovitch, Phys. Rev. Lett., vol. 58, p. 2059 (1987); S. John, Phys. Rev. Lett., vol. 58, p. 2486 (1987); Y. Fink, et al., Science, vol. 282, p. 1679 (1998); J. P. Dowling, Science, vol. 282, p. 1841 (1998); D. N. Chigrin, et al., Appl. Phys. A, vol. 68, p. 25 (1999); S. D. Hart et al., Science, vol. 296, p. 510 (2002).]) (As used herein, with respect to electromagnetic radiation incident on a generally planar surface, the term “omnidirectional reflector” indicates that the reflector is generally capable of reflecting radiation incident at any angle from 90° (normal incidence) to almost 0° and for any polarization.) More recently, hybrid devices have been proposed made of alternating layers of positive and negative refractive index materials [H. Jiang, et al., Appl. Phys. Lett., vol. 83, p. 5386 (2003); D. Bria, et al., Phys. Rev. E, vol. 69, p. 066613 (2004)].
In optics it is well known that when the inner layer of a planar waveguide is a gas with refractive index n0=1 and it is sandwiched between two standard dielectric materials with refractive index n>1, total internal reflections cannot be achieved. The field coupled inside such a waveguide attenuates in the propagation direction by leaking power to the two bounding media [A. Yariv & P. Yeh, “Optical Waves in Crystals”, John Wiley & Sons, New York, pp. 473-77 (1984)]. The losses suffered via these “leaky” modes may be balanced when the molecular gas in the core is an active medium, as, for example, in CO2 waveguide lasers [P. W. Smith, Appl. Phys. Lett., vol. 19, p. 132 (1971); T. J. Bridges, et al., Appl. Phys. Lett., vol. 20, p. 403 (1972)]. In metal-clad waveguides the refractive index of the guiding layer can be arbitrarily low as long as it is greater than the refractive index of the substrate [P. K. Tien, et al., Appl. Phys. Lett., vol. 27, p. 251 (1975) incorporated by reference]. Total internal reflections are always achieved thanks to the low refractive index of the metal. In Tien, guiding was demonstrated in an air-polystyrene -silver waveguide at optical frequencies, in a 1.81 μm thick polystyrene film. [Id.] Losses were estimated by those authors at approximately 1 dB/cm for the fundamental transverse electric (TE) mode.
The theory of hollow waveguides was developed in E. A. J. Marcatili and R. A. Schmeltzer, Bell Syst. Tech. J., vol. 43, p. 1783 (1964), and different types of hollow waveguides in the infrared have been realized over the years. Some examples are hollow sapphire fibers [J. A. Harrington and C. C. Gregory, Opt. Lett., vol. 15, p. 541 (1990)], hollow Ag/AgI coated glass waveguides [J. Dai and J. A. Harrington, Appl. Opt., vol. 36, p. 5072 (1997)], and ZnS-coated Ag hollow waveguides [Y. Matsuura and M. Miyagi, Appl. Opt., vol. 32, p. 6598 (1993)]. These guides have losses as low as 0.1 dB/m at 10.6 μm, for a bore diameter of approximately 1000 μm.
In the visible region, a tremendous breakthrough in the possibility of confining light in air was achieved in 1999, with the introduction of so-called photonic crystal fibers (PCFs) [J. C. Knight, et al., Science, vol. 282, p. 1476 (1998); R. F Cregan, et al., Science, vol. 285, p. 1537 (1999)]. In a PCF, light confinement does not require a core with a higher refractive index because guidance is achieved not by total internal reflection, but by the presence of a cladding in the form of a full two-dimensional photonic band gap.